Naphtha streams are primary petroleum refinery products. These streams are blended to makeup what is referred to in the industry as the “gasoline pool”. One problem associated with such streams, especially those naphtha streams which are products of a cracking process, such as fluidized catalytic cracking and coking, is that they contain relatively high levels of undesirable sulfur. They also contain valuable olefins which contribute to the octane number of the resulting gasoline pool, and thus it is highly desirable not to saturate them to lower octane paraffins during processing. There is a continuing need therefore, for hydrodesulfurization catalysts and processes for desulfurizing naphtha feed streams, while attempting to keep olefin saturation at a minimum.
Hydrodesulfurization that preserve olefins while removing sulfur are frequently referred to as selective hydrotreating processes. Undesirably, in selective hydrotreating a portion of the preserved olefins reacts with H2S to form mercaptans. Such mercaptans are referred to as reversion mercaptans to distinguish them from the mercaptans found in the feed to the hydrodesulfurizer. Even though two-stage hydrodesulfurization processes curtail reversion mercaptan formation through, e.g. inter-stage H2S separation, some reversion mercaptans may remain. Increasingly stringent sulfur specifications for gasoline may require still lower levels of mercaptans, including reversion mercaptans, to meet product specifications.
Mercaptans may be removed from naphtha with conventional aqueous treatment methods. In one conventional method, the naphtha contacts an aqueous treatment solution containing an alkali metal hydroxide. The naphtha contacts the treatment solution, and mercaptans are extracted from the naphtha to the treatment solution where they form mercaptide species. The naphtha and the treatment solution are then separated, and a treated naphtha is conducted away from the process. Intimate contacting between the naphtha and aqueous phase leads to more efficient transfer of the mercaptans from the naphtha to the aqueous phase, particularly for mercaptans having a molecular weight higher than about C4. Such intimate contacting often results in the formation of small discontinuous regions (also referred to as “dispersion”) of treatment solution in the naphtha. While the small aqueous regions provide sufficient surface area for efficient mercaptan transfer, they adversely affect the subsequent naphtha separation step and may be undesirably entrained in the treated naphtha.
Efficient contacting may be provided with reduced aqueous phase entrainment by employing contacting methods that employ little or no agitation. One such contacting method employs a mass transfer apparatus comprising substantially continuous elongate fibers mounted in a shroud. The fibers are selected to meet two criteria. The fibers are preferentially wetted by the treatment solution, and consequently present a large surface area to the naphtha without substantial dispersion or the aqueous phase in the naphtha. Even so, the formation of discontinuous regions of aqueous treatment solution is not eliminated, particularly in continuous processes.
In another conventional method, the aqueous treatment solution is prepared by forming two aqueous phases. The first aqueous phase contains alkylphenols, such as cresols (in the form of the alkali metal salt), and alkali metal hydroxide, and the second aqueous phase contains alkali metal hydroxide. Upon contacting the hydrocarbon to be treated, mercaptans contained in hydrocarbon are removed from the hydrocarbon to the first phase, which has a lower mass density than the second aqueous phase. Undesirable aqueous phase entrainment is also present in this method, and is made worse when employing higher viscosity treatment solutions containing higher alkali metal hydroxide concentration.
There remains a need, therefore, for improved naphtha desulfurization processes capable of efficiently removing sulfur, particularly mercaptan sulfur, without undue aqueous contamination of the treated naphtha.